The present invention describes an improved holographic process and a device using incoherent light.
The principle of incoherent holography and its application to distance measurements have been described in U.S. Pat. No. 4,602,844. This patent describes an optical system (named "Conoscopic Apparatus") including a birefringent uniaxial crystal and two polarizers which permit the hologram of a point in incoherent light to be optically formed.
If a ray of light impinges on the crystal, the birefringent properties of the crystal causes the incident light to be split into two light beams having perpendicular polarizations. The first light beam (ordinary ray) propagates inside the crystal at a velocity Vo (index of refraction No) independent of the propagation direction. The second light beam (extraordinary ray) propagates inside the crystal at a velocity Ve (index of refraction Ne) which is a function of the propagation direction.
At the first order of approximation on the incidence angle, the two light beams follow the same geometrical path in the crystal. However, because the two light beams do not propagate at the same velocity, they are out of phase with each other at the exit face of the crystal.
The polarizer placed after the crystal will permit the ordinary and extraordinary vibrations to recombine such that the phase of the resulting wave is coded in the form of an interference Figure.
The polarizer placed before the crystal permits two rays of the same intensity to be obtained such that a maximum contrast is obtained at the output.
For a small incident angle .theta., and with a crystal of length L, whose crystallographic optical axis is parallel to the optical axis of the system, the phase difference between the ordinary and extraordinary rays induced by passing through the crystal is: ##EQU1## where EQU Z.sub.0 =Z-L+(L/no) (2) EQU Z.sub.E =Z-L+(Lno/ne.sup.2) (3)
The amplitude of the wave in the observation plane is the sum of the ordinary and extraordinary amplitudes. The intensity recorded at a point Q (x, y, o) is proportional to: EQU H.sub.1 (Q)=I(P)(1+cos .DELTA..phi.) (4a)
or EQU H.sub.2 (Q)=I(P)(1-cos .DELTA..phi.) (4b)
depending on whether the two polarizers have the same handiness or not.
In these relationships, I(P) is the intensity of the wave coming from the object point P (x0,y0,z), onto the detector, i.e., this one which should be recorded without the conoscopic part.
The interference pattern obtained which represents the conoscopic hologram of a point is called Gabor zone lens.
Indeed, the conoscopic hologram of a point recorded at the wavelength .lambda. is similar to the hologram of the same point recorded with coherent light (Gabor holography) at an equivalent wavelength .lambda.eq given by: ##EQU2##
The French Patent Publication No. FR-A-2641091 describes some improvements to the system described in U.S. Pat. No. 4,602,844. Still other improvements are described in the French Patent Publication Nos. FR-A-2646251 and FR-A-2646252.
Until now, to recover the shape of the object, it was proposed to record the holograms on a CCD array or linear CCD array, to then digitize the obtained signal, and to numerically process the digitized signal. The numerical data process consists mainly of a Fourier transform.
The systems described in the above mentioned documents have a great interest. However, the known systems which include numerical data processing have a limited measurement time because of the time consumed by the numerical data processing. In addition the quantity of light needed to achieve a given signal to noise ratio (on which depends the further precision) is proportional to the square root of the number of detectors.
The object of present invention is to improve the existing devices and to suppress their disadvantages.